Electrochemical study of the tarnish layer of silver deposited on glass

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the tarnished thin layer of silver deposited on glass. Instead of natural tarnishing in air environment, an acceleration of tarnishing process was realized by immersion of Ag covered glass in 10μMK2S medium. The X-ray photoelectron spectroscopy (XPS) shows that tarnishing product formed on the silver surface consisted of Ag2S and Ag2O. As electrochemical characterization, the measurements were carried out in aerated 0.5M Na2SO4 solution adjusted at pH 10. The impedance spectra collected in sulphate medium at the open-circuit potential show one capacitive loop in parallel with a high resistance, which reflects a blocking electrode behaviour. However, the equivalent electrical circuit, Rs-(CPE//R) is insufficient to reproduce the experimental results correctly. To minimize the dispersion between the experimental and fitted data, the CPE contribution is replaced by two normal power-law distributions of the local resistivity to interpret the tarnishing process in K2S medium with respect to the immersion time. These distributions are associated with the Ag2S and Ag2O layers.